U.S. patent application number 13/396609 was filed with the patent office on 2012-06-14 for dc/ac inverter.
Invention is credited to Shih-Chung Huang, Chien-Pang Hung, Chih-Shun Lee, Chung-Che Yu.
Application Number | 20120146534 13/396609 |
Document ID | / |
Family ID | 36180554 |
Filed Date | 2012-06-14 |
United States Patent
Application |
20120146534 |
Kind Code |
A1 |
Yu; Chung-Che ; et
al. |
June 14, 2012 |
DC/AC Inverter
Abstract
A fluorescent lamp device includes a frequency generator for
generating a pulse signal, a driver circuit coupled to said
frequency generator for generating at least one driving signal
according to said pulse signal, a half bridge power switch unit
coupled to the driver circuit, a resonant tank coupled to the half
bridge power switch unit, and a fluorescent lamp coupled to the
resonant tank.
Inventors: |
Yu; Chung-Che; (Taipei,
TW) ; Huang; Shih-Chung; (Taipei, TW) ; Hung;
Chien-Pang; (Taipei, TW) ; Lee; Chih-Shun;
(Taipei, TW) |
Family ID: |
36180554 |
Appl. No.: |
13/396609 |
Filed: |
February 15, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13042472 |
Mar 8, 2011 |
8143797 |
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13396609 |
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12693401 |
Jan 25, 2010 |
7952296 |
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13042472 |
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11580186 |
Oct 13, 2006 |
7737642 |
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12693401 |
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10968857 |
Oct 18, 2004 |
7148633 |
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11580186 |
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Current U.S.
Class: |
315/224 ;
363/97 |
Current CPC
Class: |
H05B 41/2825 20130101;
Y02B 20/00 20130101; H05B 41/2828 20130101; Y02B 70/1441 20130101;
Y02B 20/186 20130101; Y02B 70/10 20130101; H02M 7/53871
20130101 |
Class at
Publication: |
315/224 ;
363/97 |
International
Class: |
H05B 41/36 20060101
H05B041/36; H02M 7/44 20060101 H02M007/44 |
Claims
1. A fluorescent lamp device comprising: a fluorescent lamp; a
frequency generator for generating a pulse signal; a driver circuit
coupled to said frequency generator for generating at least one
driving signal according to said pulse signal; a half bridge power
switch unit coupled to said driver circuit for generating an AC
signal according to said at least one driving signal; and a
resonant tank electrically connected between said half bridge power
switch unit and said fluorescent lamp for filtering said AC signal
to supply power to said fluorescent lamp.
2. The fluorescent lamp device of claim 1, wherein said half bridge
power switch unit comprises a first switch coupled to a direct
current (DC) voltage line and a second switch coupled to
ground.
3. The fluorescent lamp device of claim 1, further comprising: a
protection circuit electrically connected to said fluorescent lamp
for stably adjusting voltage of said fluorescent lamp.
4. The fluorescent lamp device of claim 3, wherein said protection
circuit comprises: a comparator for comparing a signal indicating a
terminal voltage of said fluorescent lamp with a reference voltage
to generate a comparison signal to decrease said supply power when
said signal exceeds said reference voltage.
5. The fluorescent lamp device of claim 4 further comprising a
pulse width modulator electrically connected to said driver circuit
for providing a pulse width modulation signal to said driver
circuit according to said comparison signal, so as to generate said
at least one driving signal.
6. The fluorescent lamp device of claim 5, wherein said pulse width
modulator comprises: an error amplifier; a resistor electrically
connected to an inverting input of said error amplifier; a
capacitor having one terminal electrically connected to said
resistor and said inverting input of said error amplifier to form
an integrator; a comparator having a non-inverting input
electrically connected to an output of said error amplifier and the
other terminal of said capacitor; a switch controlled at least by
said comparison signal; and a controlled current source
electrically connected to said inverting input of said error
amplifier through said switch.
7. The fluorescent lamp device of claim 5, further comprising a
dimming control circuit electrically connected to said frequency
generator for generating a dimming control pulse signal to control
said pulse width modulator to provide said pulse width modulation
signal or not.
8. The fluorescent lamp device of claim 7, wherein said dimming
control circuit comprises: a dimming control frequency generator
for generating a triangular signal; and a comparator having a
non-inverting input electrically connected to said dimming control
frequency generator for receiving said triangular signal and an
inverting input electrically connected to a dimming control
voltage, wherein said comparator compares said dimming control
voltage and said triangular signal to generate said dimming control
pulse signal.
9. The fluorescent lamp device of claim 1, further comprising: a
protection circuit electrically connected to said fluorescent lamp
for disabling said first power switch and said second power switch
when no fluorescent lamp is electrically connected thereto.
10. The fluorescent lamp device of claim 9, wherein said protection
circuit comprises: a logic control circuit for generating a
terminating signal to disable said first power switch and said
second power switch; and a comparator for comparing a signal
indicating a terminal voltage of said fluorescent lamp with a
reference voltage to generate a comparison signal so as to generate
said terminating signal.
11. A direct current to alternating current (DC/AC) inverter for
inverting a DC voltage source to an AC power source to drive a
load, comprising: a frequency generator for generating a pulse
signal; a driver circuit coupled to said frequency generator for
generating at least one driving signal according to said pulse
signal; a half bridge power switch unit coupled to said driver
circuit for generating an AC signal according to said at least one
driving signal; and a resonant tank electrically connected between
said half bridge power switch unit and said load for filtering said
AC signal to supply power to said load.
12. The DC/AC inverter of claim 11, wherein said half bridge power
switch unit comprises a first switch coupled to a direct current
(DC) voltage line and a second switch coupled to ground.
13. The DC/AC inverter of claim 11, further comprising: a
protection circuit for electrically connecting to said load for
stably adjusting voltage of said load.
14. The DC/AC inverter of claim 13, wherein said protection circuit
comprises: a comparator for comparing a signal indicating a
terminal voltage of said load with a reference voltage to generate
a comparison signal to decrease said supply power when said signal
exceeds said reference voltage.
15. The DC/AC inverter of claim 14 further comprising a pulse width
modulator electrically connected to said driver circuit for
providing a pulse width modulation signal to said driver circuit
according to said comparison signal, so as to generate said at
least one driving signal.
16. The DC/AC inverter of claim 15, wherein said pulse width
modulator comprises: an error amplifier; a resistor electrically
connected to an inverting input of said error amplifier; a
capacitor having one terminal electrically connected to said
resistor and said inverting input of said error amplifier to form
an integrator; a comparator having a non-inverting input
electrically connected to an output of said error amplifier and
said other terminal of said capacitor; a switch controlled at least
by said comparison signal; and a controlled current source
electrically connected to said inverting input of said error
amplifier through said switch.
17. The DC/AC inverter of claim 15, further comprising a dimming
control circuit electrically connected to said frequency generator
for generating a dimming control pulse signal to control said pulse
width modulator to provide said pulse width modulation signal or
not.
18. The DC/AC inverter of claim 17, wherein said dimming control
circuit comprises: a dimming control frequency generator for
generating a triangular signal; and a comparator having a
non-inverting input electrically connected to said dimming control
frequency generator for receiving said triangular signal and an
inverting input electrically connected to a dimming control
voltage, wherein said comparator compares said dimming control
voltage and said triangular signal to generate said dimming control
pulse signal.
19. The DC/AC inverter of claim 11, further comprising: a
protection circuit electrically connected to said load for
disabling said first power switch and said second power switch when
no load is electrically connected thereto.
20. The DC/AC inverter of claim 19, wherein said protection circuit
comprises: a logic control circuit for generating a terminating
signal to disable said first power switch and said second power
switch; and a comparator for comparing a signal indicating a
terminal voltage of said load with a reference voltage to generate
a comparison signal so as to generate said terminating signal.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/042,472, filed Mar. 8, 2011, which is a
continuation of U.S. patent application Ser. No. 12/693,401, filed
Jan. 25, 2010, which is a continuation of U.S. patent application
Ser. No. 11/580,186, filed Oct. 13, 2006, now U.S. Pat. No.
7,737,642, which is a continuation-in-part of U.S. patent
application Ser. No. 10/968,857, filed Oct. 18, 2004, now U.S. Pat.
No. 7,148,633, all of which are incorporated herein in their
entirety by reference.
BACKGROUND OF THE PRESENT INVENTION
[0002] 1. Field of Invention
[0003] The present invention relates to a DC/AC inverter, and more
particularly to a DC/AC inverter for driving a background
fluorescent lamp of an LCD, especially for a plurality of power
operated half-bridge DC/AC inverter designed for driving a
plurality of fluorescent lamps.
[0004] 2. Description of Related Arts
[0005] As liquid crystal displays (LCD) thinner than conventional
cathode ray tube (CRT) monitors, they are being used in more and
more homes and public display. However, since LCD is operated by
its optical rotary power and optical characteristic to display
image and text information, hence not illuminable, it requires an
additional backlight source. An example of backlight source for LCD
is fluorescent lamps.
[0006] A typical DC/AC inverter utilizes a full-bridge inverter
circuit, along with a resonant tank and a step-up transformer, a DC
power input can be filtered and converted to a high AC voltage by
the resonant tank and the step-up transformer, so as to drive the
fluorescent lamp.
[0007] In order to stabilize the light emitted by the fluorescent
lamp, and prevent a change in light intensity while there is a
change in the power input voltage, most inverters are incorporated
with negative feedback circuits for stabilizing the current in the
fluorescent lamp. As the life-span of the fluorescent lamp is
affected by the symmetry of the waveform of the current, it is most
popular to use full-bridge inverter to drive fluorescent lamps.
[0008] Referring to FIG. 1 of the drawing, a conventional
full-bridge inverter is illustrated. As shown in FIG. 1, a
full-bridge inverter 100 comprises a DC voltage source 101, a
full-bridge switch circuitry 102, a resonant tank 103, a
fluorescent lamp 104, a current sensing circuit 105, a pulse width
modulator 106, a frequency generator 107, and a full-bridge switch
driver circuit 108, wherein the full-bridge switch circuitry 102
comprises four power switches 101A, 101B, 101C and 101D. The
resonant tank 103 comprises a step-up transformer 120 and two
resonant capacitors 121 and 122. The frequency generator 107
comprises a triangular wave generator 110 and a pulse generator
109. The full-bridge switch driver circuit 108 provides four sets
of driving signal R1, R2, R3 and R4.
[0009] The DC voltage source 101 is electrically connected to the
full-bridge switch circuitry 102, wherein the output of the
full-bridge switch circuitry 102 is electrically connected to an
input of the resonant tank 103. An output of the resonant tank 103
is electrically connected to a terminal of the fluorescent lamp
104. The series connection between the full-bridge switch circuitry
102, the resonant tank 103 and the fluorescent lamp 104 is a
typical example of a power transfer connection.
[0010] The current sensing circuit 105 is electrically connected to
the fluorescent lamp 104 and the pulse width modulator 106. The
pulse width modulator 106 is then electrically connected to the
frequency generator 107 and the full-bridge switch driver circuit
108, which is electrically connected to the gate terminals of the
full-bridge switch circuitry 102, forming a control loop
connection.
[0011] Conventional full-bridge inverter is operated based on the a
fixed high frequency conduction between the four power switches of
the full-bridge switch circuitry 102, such that the DC voltage
output by the DC voltage source 101 is transformed to and outputted
as a fixed high-frequency AC square wave, which is provided for
being inputted to the resonant tank 103. The resonant tank 103
utilizes the step-up characteristic and the filter function of the
step-up transformer 120 to transform the fixed high-frequency AC
square wave to a fixed high frequency AC sine wave, which is
provided to the fluorescent lamp 104.
[0012] The control loop utilizes the current sensing circuit 105 to
produce a feedback signal R5, which corresponds to a fluorescent
lamp current, which is then transferred to the pulse width
modulator 106. The pulse width modulator 106, together with the a
triangular wave output R6 by the triangular wave generator 110 of
the frequency generator 107, utilizing the theory of negative
feedback, produces a pulse width modulation signal R7 for inputting
to the full-bridge switch driver circuit 108, wherein the
full-bridge switch driver circuit 108 utilizes the pulse width
modulation signal R7 and the frequency generator 107 to produce the
four sets of driving signals R1, R2, R3 and R4 so as to drive the
four power switches 101A, 101B, 101C and 101D.
[0013] By controlling the conduction period between the two power
switches 101A and 101D, and the conduction period between the two
power switches 101C and 101B, the alternating conduction between
101A and 101D, and 101C and 101B provides a stable fluorescent lamp
current which is an AC current having a symmetrical waveform.
[0014] This conventional type of full-bridge inverter circuits can
stably control the current of a fluorescent lamp, however, has the
draw back of having a great number of switches, pushing the
production cost of such circuits higher.
[0015] As a result, the present invention is to provide a cheaper
and more reliable DC/AC inverter.
SUMMARY OF THE PRESENT INVENTION
[0016] A main object of the present invention is to provide a
circuitry of a DC/AC inverter for driving a fluorescent lamp
circuit, wherein the DC/AC inverter uses less power switches to
control the operation of the fluorescent lamp, producing lower DC
voltage ripple, which in turn lowers noise caused by system
ripples.
[0017] Another object of the present invention is to provide the
circuitry of a half-bridge DC/AC inverter, having an advantage of
using less power switches and lower production cost, together with
an alternating operation to achieve lower DC voltage ripple, which
in turn lowers noise caused by system ripples when multi
fluorescent lamps are operated.
[0018] Another object of the present invention is to provide two
sets of power switch driving signals of the DC/AC inverter, such
that the duty cycle of each of the two sets of power switch driving
signals alters symmetrically with respect to that of the other
power switch driving signals. Since the power switches do not
conduct simultaneously upon receiving the DC voltage, the noise of
the DC voltage source is minimized.
[0019] Another object of the present invention is to provide a
plurality of sets of power switch driving signals of the DC/AC
inverter, which is applied to a plurality of fluorescent lamps,
such that the fluorescent lamps can utilize frequency generators to
generate a plurality of signals with identical frequency and
different phases as frequency sources.
[0020] Accordingly, in order to accomplish the above objects, the
present invention provides a DC/AC inverter for transforming a DC
power source to an AC power source, an AC signal of which is used
to drive a fluorescent lamp, wherein the DC/AC inverter comprises:
[0021] a half-bridge switch circuitry electrically connected to the
DC power source; [0022] a resonant tank electrically connected
between the half-bridge switch circuitry and the fluorescent lamp,
wherein the resonant tank steps-up and filters the AC signal from
the half-bridge switch circuitry, such that the AC signal is
transformed to the AC power source; and [0023] a controller feeding
back an output of the fluorescent lamp, providing a pulse width
modulation signal to control, the conduction of the half-bridge
switch circuitry so as to drive the fluorescent lamp being operated
around the resonant frequency of the fluorescent lamp according to
the conduction condition of the half-bridge switch circuitry.
[0024] These and other objectives, features, and advantages of the
present invention will become apparent from the following detailed
description, the accompanying drawings, and the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 illustrates a prior art of a full-bridge fluorescent
lamp inverter circuitry.
[0026] FIG. 2 illustrates a circuitry of a DC/AC inverter according
to a preferred embodiment of the present invention.
[0027] FIG. 3 illustrates some waveforms of the circuitry of the
DC/AC inverter according to the above preferred embodiment of the
present invention.
[0028] FIG. 4 illustrates a circuitry of two DC/AC inverters
according to an alternative embodiment of the present
invention.
[0029] FIG. 5 illustrates a sequential marked graph according to
the above alternative embodiment of the present invention.
[0030] FIG. 6 illustrates a circuitry of a plurality of DC/AC
inverters according to yet another alternative embodiment of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0031] Referring to FIG. 2 of the drawings, a circuitry of a DC/AC
inverter according to the preferred embodiment of the present
invention is illustrated, wherein the DC/AC inverter is a
half-bridge DC/AC inverter.
[0032] As shown in the drawing, an inverter controller 200
comprises a DC voltage source 201, a half-bridge switch circuitry
202, a resonant tank 203, a fluorescent lamp 204, a current sensing
circuit 205 for sensing a lamp current of the fluorescent lamp 204,
a voltage sensing circuit 206 for sensing a terminal voltage of the
fluorescent lamp 204, a pulse width modulator 207, a frequency
generator 208, a driver circuit 209, which is a half-bridge switch
driver circuit, a protection circuit 210, a timer 211, and a
dimming control circuit 212.
[0033] The DC voltage source 201 is electrically connected to the
half-bridge switch circuitry 202, one of the switch 202A is
connected to a DC voltage line and the other one 202B is connected
to the ground, wherein through an output of the half-bridge switch
circuitry 202, the half-bridge switch circuitry 202 is electrically
connected to the resonant tank 203 though an input of the resonant
tank 203. Through an output of the resonant tank, the resonant tank
203 is also electrically connected to the fluorescent lamp 204. The
resonant tank 203 further comprises a step-up transformer 221 and
resonant capacitors 222, 223, and 224.
[0034] The electrical connection method between the elements of the
DC/AC inverter is conventionally known as power transfer
connection. According to the preferred embodiment of the present
invention, a Low Q resonant tank is used so as to provide easy
designing of the circuitry.
[0035] Under such a circuitry design, the waves that drive the
step-up transformer 221, and the fluorescent lamp 204 is quasi sine
waves or quasi square waves, as oppose to pure square waves or pure
sinusoidal waves. FIG. 3 of the drawings illustrates the voltage
wave form at different positions under the power transmission route
design according to the preferred embodiment.
[0036] In FIG. 3 of the drawings, S51 is the voltage waveform of
the output end of the half-bridge switch circuitry 202 and S16 is
the voltage waveform of the driving signal output by the step-up
transformer 221.
[0037] The current sensing circuit 205 and the voltage sensing
circuit 206 are electrically connected to the ends of the
fluorescent lamp 204 respectively. The current sensing circuit 205
is also electrically connected to the pulse width modulator 207,
wherein the pulse width modulator 207 is also electrically
connected to the frequency generator 208 and the driver circuit
209. The driver circuit 209 is in turn electrically connected to
the half-bridge switch circuitry 202, forming a control loop
connection.
[0038] The half-bridge switch circuitry 202 comprises two power
switches 202A and 202B. According to the preferred embodiment of
the present invention, the power switch 202A connected to the
voltage line is a P-type MOSFET, while the power switch 202B
connected to ground line is an N-type MOSFET. However, the power
switches are not limited to MOSFET, and can also be another
semiconductor switches, such as BJT switches.
[0039] The frequency generator 208 generates a triangular wave
signal S1 and a pulse signal S2, wherein both signals have the same
frequency. However, the present invention is not limited to the use
of triangular wave signals, where all ramp signals and sawtooth
wave signals are applicable.
[0040] The current sensing circuit 205 and the fluorescent lamp 204
are in series to provide a signal S3 for indicating the conduction
of the fluorescent lamp 204, and utilize a second signal S4 to show
the current value of the current flowing through the fluorescent
lamp 204. The voltage sensing circuit 206 utilizes the resonant
capacitors 223 and 224 of the resonant tank 203 to obtain a third
signal S5 for indicating a terminal voltage of the fluorescent lamp
204.
[0041] The pulse width modulator 207 comprises an error amplifier
261, a resistor 262, and a capacitor 263, and a comparator 264. The
pulse width modulator 207 also comprises a controlled current
source 265, which is electrically connected to an inverse input of
the error amplifier 261 through a switch 266.
[0042] The driver circuit 209 comprises two driving signals POUT
and NOUT. The protection circuit 210 comprises a logic control
circuit 272. The protection circuit 210 receives the signal S3
capable of indicating the conduction state of the fluorescent lamp,
the third signal S5 capable of indicating the terminal voltage of
the fluorescent lamp, and an output signal S6 of the error
amplifier 261 in the pulse width modulator 207.
[0043] The timer 211 comprises two comparators 281 and 282, and a
current source 283. The dimming control circuit 212 comprises a
dimming control frequency generator 291, wherein a triangular
signal S7 and a dimming control voltage S8 are generated by the
dimming control frequency generator 291. The triangular signal S7
is delivered to a non-inverting input of a comparator 293, and the
dimming control voltage S8 is delivered to an inverting input of
the comparator 293. After comparison, a dimming control pulse
signal S9 is generated, the dimming control circuit 212 further
comprises an OR gate 296 for controlling the timing of the
outputting of the dimming control pulse signal S9 to the pulse
width modulator 207.
[0044] According to the preferred embodiment of the present
invention, the timer 211 functions in a manner such that a timer
capacitor 284 is being charged by the current source 283, such that
a voltage S12 of the timer capacitor 284 increases as time
increases. Before the voltage S12 reaches a first reference voltage
Vref1, a reset signal S11 is being sent out. When the voltage S12
reaches a second reference voltage Vref2, a time out signal S10
will be sent out.
[0045] The current source 283 is being controlled by a signal S13
capable of indicating the system voltage, such that when the system
voltage is lower than a third reference voltage Vref3, the current
source 283 will be cut off, and the voltage S12 of the timer
capacitor 284 earthed. Through such design, every time when the
system starts up the DC voltage source 201 from a zero voltage, the
timer capacitor 284 of the timer 211 is charged started from zero
voltage.
[0046] According to the preferred embodiment of the present
invention, the frequency generator 208 is also controlled by a
fourth signal S14, indicating whether or not the fluorescent lamp
is conducted. When the fluorescent lamp is conducted, an operation
frequency is sent out. When the fluorescent lamp is not conducted,
a start-up frequency is sent out.
[0047] An advantage of such a design is that the resonant frequency
of the resonant tank 203 can operated by the different operation
frequencies according to the conduction state of the fluorescent
lamp, such that the system can be operated around the resonant
frequency, whether or not the fluorescent lamp is conducted, such
that the system is operated efficiently.
[0048] The fourth signal S14 is determined by the signal S3
provided by the current sensing circuit 205, and a comparator 274
of the protection circuit 210, such that when the signal S3 exceeds
a fourth reference voltage of Vref4, the fluorescent lamp 204 is
considered as being conducted.
[0049] Under normal circumstances, the detailed operation
procedures according to the preferred embodiment of the present
invention are as follows:
[0050] After staring up to supply voltage to the system, the timer
211 initializes the charging of the timer capacitor 284, such that
before the voltage of the timer capacitor 284 reaches the first
reference voltage Vref1, the reset signal S11 being sent out by the
timer 211 passes through an OR gate 267, turning on a switch 266,
such that the current source 265 is connected to the inverting
input of the error amplifier 261, forcing an input voltage of the
inverting input of the error amplifier 261 to be higher than a
fifth reference voltage Vref5, which in turn forces an error output
S6 to be zero.
[0051] When the timer capacitor 284 continue to charge until the
capacitor voltage is greater than the first reference voltage
Vref1, the switch 266 will be turned off, such that the pulse width
modulator 207 is initialized, and because of the fluorescent lamp
204 is not conducted, the inverting input voltage of the error
amplifier 261 becomes lower than the fifth reference voltage Vref5,
causing the output signal S6 outputted by the error amplifier 261
to increase under the effect of negative feedback.
[0052] After comparing with the triangular wave signal S1, the
comparator 264 of the pulse width modulator 207 outputs a pulse
width modulation signal S15. The driver circuit 209 receives the
pulse width modulation signal S15 and the pulse signal S2 so as to
produce the driving signals POUT and NOUT to drive the power switch
202A and 202B respectively.
[0053] Before the conduction of the fluorescent lamp 204, the
voltage S16 of the fluorescent lamp 204 will increase due to a
widening of duty cycle of the pulse width modulation signal S15.
Upon sensing the third signal S5 exceeding a sixth reference
voltage Vref6, the voltage sensing circuit 206 sends out an over
voltage signal S17, which passes through the OR gate 267 and turn
on the switch 266 of the current source 265, to the inverting input
of the error amplifier 261, so as to reduce the output signal S6 of
the error amplifier 261, whereby decreasing the duty cycle of the
pulse width modulation signal S15 and decreasing the amount of
electrical power input to the fluorescent lamp.
[0054] If the effect of this decreasing in amount of electrical
power input to the fluorescent lamp is a sensing of the third
signal S5 to be less than the sixth reference voltage Vref6, the
switch 266 will be turned off, increasing the output signal S6 of
the error amplifier 261. As a result, the voltage S16 of the
fluorescent lamp 204 is stably adjusted under such a negative
feedback control.
[0055] As soon as the fluorescent lamp is conducted by a sufficient
voltage S16 of the fluorescent lamp 204 and for a substantial
period of time, according to the characteristic of the fluorescent
lamp, the voltage S16 of the fluorescent lamp 204 will drop to less
than half the voltage required for conducted operation, such that
the voltage sensing circuit 206 loses its function due to a
non-detection of a higher voltage.
[0056] At the same time, the current sensing circuit 205 sends out
the signal S3 to the protection circuit 210, producing the fourth
signal S14 to alter the output frequency of the frequency generator
208, and outputting the second signal S4 to the pulse width
modulator 207, such that the current flowing through the
fluorescent lamp is stabilized on a pre-determined value through
the negative feedback control.
[0057] According to the preferred embodiment of the present
invention, the protection circuit is operated as follows:
[0058] When the fluorescent lamp is not connected, the third signal
S5 will continuously send out a signal indicating that the terminal
voltage of the fluorescent lamp exceeds the sixth reference voltage
Vref6 to the logic control circuit 272, which receives the time out
signal S10 of the timer 211.
[0059] The logic control circuit 272 will take no action until the
time out signal S10 is inputted. Once the time out signal S10
reaches the logic control circuit 272, and in the condition of the
terminal voltage of the fluorescent lamp exceeds the sixth
reference voltage Vref6, it times with another digital timer (not
shown in the diagram), by the pulse signal S2 produced by the
frequency generator 208 to time.
[0060] If the terminal voltage of the fluorescent lamp still
exceeds the sixth reference voltage Vref6 after a predetermined
period of time, a terminating signal S18 will be outputted by the
logic control circuit 272 to the driver circuit 209, so as to cut
off the conduction of the power switches 202A and 202B.
[0061] If the fluorescent lamp is damaged during operation, the
fourth signal S14 will be sent out, indicating that the fluorescent
lamp is not conducted, to the logic control circuit 272, receiving
the time out signal S10 from the timer 211. The logic control
circuit 272 will not take any action until the receiving the time
out signal S10.
[0062] When time is up, the logic control circuit 272, under the
condition of the fourth signal S14 indicating that the fluorescent
lamp is not conducted, will time with a second digital timer,
through a second pulse signal S21 produced by the low frequency
dimming control circuit 212.
[0063] If the lamp still is not conducted after a predetermined
period of time, the logic control circuit 272 will output the
terminating signal S18 to the driver circuit 209, so as to cut off
the conduction of the power switches 202A and 202B.
[0064] Also, when, the step-up transformer 221 encounters serious
damage, such as power leakage, creating an overloading effect, the
entire system will be overloaded. Under such conditions, the error
amplifier 261 will continue to increase its outputting of the
output signal S6, so as to provide sufficient power to stabilize
the current of the fluorescent lamp. If the leakage is greater than
the maximum power provided by the system, the error amplifier 261
will definitely exceed the peak value of the triangular wave signal
S1.
[0065] The protection circuit 210 compares the output signal S6 of
the error amplifier 261 with a seventh reference voltage Vref7, the
value of which is slightly higher than the peak value of the
triangular wave signal, to obtain an overloading signal S19,
indicating whether or not the system is overloaded.
[0066] Similarly, if the overloading signal S19 indicates that the
system is overloaded when the timer 211 initializes the protection
circuit 210, and, if the timing using the pulse signal S2, which
passes through the logic control circuit 272 and is produced by the
frequency generator 208, also exceeds the predetermined period of
time, the logic control circuit 272 then outputs the terminating
signal S18 to the driver circuit 209, cutting off the conduction of
the power switches 202A and 202B.
[0067] According to the preferred embodiment of the present
invention, the inverter further has the dimming control circuit 212
provided for controlling the termination and restarting of the
power provided to the fluorescent lamp. It makes use of the
adjusting of brightness ratio to adjust the brightness of the
fluorescent lamp. In order to avoid the creating of the flashing
feeling created by a low frequency, the brightness frequency is
normally controlled to be above 200 Hz.
[0068] The dimming control circuit is controlled by two signals,
the first one being the fourth signal S14 indicating whether or not
the fluorescent lamp is conducted, and the second one being the
time out signal S10 of the timer 211. Only when the fourth signal
S14 indicates that the fluorescent lamp is conducted or the timer
211 receives the time out signal S10, a switch 236 controlling the
output of the dimming control signal will be turned on.
[0069] A dimming control voltage S20 of the dimming control circuit
is higher than the fifth reference voltage Vref5. When the dimming
control voltage S20 passes through the switches 235 and 236 to be
connected to a second resistor 234 and the pulse width modulator
209, the output signal S6 of the error amplifier 261 of the pulse
width modulator 297 is decreased, cutting off electricity
transferring of the system to avoid overloading.
[0070] When the switch 235 is turned off by the dimming control
pulse signal S9, the pulse width modulator 207 is reopened and
restarting to provide electrical power to the system.
[0071] Dimming control effect can be achieved by a low frequency to
control the ratio between the stopping and the restarting of
providing electrical power of each cycle. In order to ensure that
the fluorescent lamp has sufficient and continuous electrical power
so as to be ignited in a predefined period of time, the time when
brightness can be adjusted is determined by whether or not the
fluorescent lamp is conducted.
[0072] In order to provide an AC current with good symmetry to
drive the fluorescent lamp 204, according to the preferred
embodiment of the present invention, when the system is operated
steadily, the half-bridge switch circuitry 202 is alternately
conducted with the same duty cycle, but shifted by 180 degrees.
[0073] Referring to FIG. 4 of the drawings, an alternative
embodiment of the present invention is illustrated, wherein two
sets of DC/AC inverters are operated and applied to two fluorescent
lamps simultaneously. The elements in each of a first inverter set
301 and a second inverter set 302 is substantially the same as that
of the DC/AC inverter as shown in FIG. 2 of the drawings.
[0074] It should be noted that a timer 303 is shared by the first
and the second inverter set 301 and 302, and, in order to be
applicable to the two sets of inverters according to this
embodiment of the present invention, a frequency generator 304 and
a brightness adjusting circuit 305 must be appropriately
altered.
[0075] A frequency control signal T3 of the frequency generator 304
determines when a change in frequency is required, according to a
first conduction confirmation signal T1 (similar to S14 of FIG. 2)
and a second conduction confirmation signal T2 of the first and the
second inverter set 301 and 302 respectively, and a timer signal T4
of the timer 303.
[0076] After passing the conduction confirmation signals T1 and T2
through an AND gate 311, a third conduction confirmation signal T5
is obtained. After passing the third conduction confirmation signal
T5 and the timer signal T4 though an OR gate 312, the frequency
control signal T3 is obtained.
[0077] An operation frequency of the frequency generator 304 will
be changed after the fluorescent lamps are all conducted or the
timer signal T4 outputted by the timer 303. As a result, the
frequency of the system can still be altered even when one of the
fluorescent lamps is damaged.
[0078] The frequency control signal T3 is also used for controlling
the dimming control circuit 305. The outputting moment of the
dimming control circuit 305 for adjusting the brightness is also
after the conduction of the fluorescent lamps, or after the timer
303 outputted the timer signal T4. As a result, not only can it be
ensured that both lamps are successfully lit up, brightness
adjustment can still be achieved even when one of the lamps is
damaged.
[0079] A second pulse signal T7 is produced when a first pulse
signal T6, which is outputted by the frequency generator 304 to the
DC/AC inverter, passes through an inverter 313. Utilizing the first
pulse signal T6 and the second pulse signal T7 having the same
frequency as but out of phase of the first pulse signal T6, such
that the first DC/AC inverter outputs a first set of driving
signals POUT1 and NOUT1, for driving a first power switch P1 and
N1.
[0080] The second DC/AC inverter outputs a second set of driver out
signals POUT2 and NOUT2 for driving a second power switch P2 and
N2, wherein the second set of driving signals has the same
frequency as but out of phase of the first set of driving
signals.
[0081] Referring to FIG. 5 of the drawings, a sequential marked
graph of the driving signals according to this alternative
embodiment of the present invention is illustrated. The dashed
portion of FIG. 5 shows the change in duty cycle of the driving
signals POUT1, NOUT1 and POUT2, NOUT2.
[0082] In order to keep the symmetry of the lamp driving current,
the change between the duty cycle between the driving signals
POUT1, NOUT 1 and POUT2, NOUT2 is symmetrical. Since the driving
signals POUT1 and POUT2 will not be conducted simultaneously, a
voltage noise of the power source will be reduced.
[0083] Referring to FIG. 5 of the drawings, because POUT1 and POUT2
is out of phase by 180 degrees, the current flowing into the
fluorescent lamps 318 and 319 will be reversed. Also, by adjusting
the polarity of the transformers 321 and 322, the current flowing
into the fluorescent lamp 318 and 319 can be altered to be in
phase.
[0084] When more than two fluorescent lamps are in use, a plurality
of frequency generators, each having the same frequency but out of
phase with each other, is used as a frequency source to drive the
fluorescent lamp.
[0085] Referring to FIG. 6 of the drawings, a plurality of
frequency generators provided for driving N number of DC/AC
inverters according to this alternative embodiment of the present
invention is illustrated. The input of the plurality of frequency
generators 501 can be an external clock pulse 502, which can be any
one frequency related to the control signal of the LCD display. The
other input is a frequency control signal 503.
[0086] The frequency control signal 503 utilizes an AND gate 504
and an OR gate 505 to control the changing of the operation
frequency of the fluorescent lamp, according to the conduction
confirmation signals 506, 507, 508 . . . N, confirming whether or
not all the N number of fluorescent lamps are conducted, or upon
the outputting of a timer signal 509 by the timer.
[0087] Each of the triangular wave signal 510 output by the
plurality of frequency generators 501 to each of the pulse width
modulator has the same frequency but out of phase. Each of the
pulse signal 511 output to each of the switch driver circuit has
the same frequency as and in phase with the triangular wave signal
510.
[0088] After the dimming control signal 531 entering the plurality
of frequency generators 501, the plurality of frequency generator
501 generates a dimming control pulse signal 532 that is produced
with a frequency that is relative to the LCD control frequencies,
such that the frequency control signal 503 controls the switch 533,
so as to control when the dimming control pulse signal 532 is
outputted to each of the DC/AC inverters.
[0089] Such a plurality of frequency generator 501 can be achieved
by the use of a conventional micro control unit (MCU) 521, together
with a direct digital synthesizer (DDS) 522.
[0090] Due to the fact that not the power switch of the DC voltage
source will not be conducted all at the same time, as oppose to
conventional circuitry, noises related to the power source is
minimized. And since the operation frequency is synchronized with
the LCD controller, visual disturbance due to interference caused
by frequency difference can be minimized too.
[0091] The usage of the outputting method of the frequency
generator is not limited to half-bridge DC/AC inverters. When there
are more than two sets of fluorescent lamp, this outputting method
can also be applied full-bridge or other control systems that use
the same frequency, so as to minimize noises related to the power
source and visual disturbance. Also, the present invention utilizes
fluorescent lamp to illustrate the preferred embodiment, but its
application should not be limited to fluorescent lamp. The present
invention as disclosed above can be applied to any lighting
element.
[0092] One skilled in the art will understand that the embodiment
of the present invention as shown in the drawings and described
above is exemplary only and not intended to be limiting.
[0093] It will thus be seen that the objects of the present
invention have been fully and effectively accomplished. It
embodiments have been shown and described for the purposes of
illustrating the functional and structural principles of the
present invention and is subject to change without departure from
such principles. Therefore, this invention includes all
modifications encompassed within the spirit and scope of the
following claims.
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